1. Field of the Invention
This invention relates to a quality paving asphalt cement composition, a process for its manufacture, and its use in paving operations. The composition is manufactured by treating propane-precipitated asphalt (PPA), also termed "propane deasphalting unit fraction" (PDU), with elemental sulfur at a temperature of about 140.degree. to 300.degree. C. until the evolution of hydrogen sulfide ceases. The resulting sulfur-treated asphalt base is blended with a liquid petroleum fraction, having an atmospheric equivalent boiling range of about 370.degree. to 500.degree. C., e.g., vacuum gas oil or an extract oil, producing an asphalt cement composition having improved temperature susceptibility and ductility characteristics and meeting ASTM standards.
2. Description of the Prior Art
Asphalt formulations comonly termed "asphalt," or "bitumen," which are used in road paving operations are blends of an "asphalt cement," i.e., a binder material, and inorganic mineral aggregate, such as stone, sand and/or mineral filler.
Asphalt cement prepared from heavy crude oil is normally a "straight run residuum, " being the residue produced after distilling off volatile petroleum fractions from the asphalt-bearing crude oil. Normally, this residuum is a liquid which can be "run" out of the distillation process at elevated temperatures and, after adjustment for consistency, is suitable for directly manufacturing asphalt cement formulations.
However, it is often necessary to adjust the composition of the residuum in order to meet strict paving grade asphalt specifications. This is particularly true of asphalts prepared from light crudes having relatively high API gravities, such as Arabian Light which is a large source of asphalt-bearing crude oil. Normally, API gravities (measured at 15.5.degree. C.) for medium and heavy crudes is in the vicinity of 20.degree. and lower, whereas for light crudes, including Arabian Light, the gravities are about 30.degree. and higher. A residuum from these light crudes is always marginal for asphalt manufacture because it is not directly obtainable as a "straight run residumm" for producing acceptable paving grade asphalt cements, for example, ASTM paving grades AC-21/2, AC-5, AC-10, AC-20, and AC-40, particularly AC-10 and AC-20, which meet the rigid specifications of ASTM D 3381-76, specifically Table 2, or AASHTO M 226-73, specifically Table II. Paving grades meeting these specifications possess excellent temperature susceptibility and ductility properties as required, for example, in the United States and Australia.
By the term, "temperature susceptibility," as used herein, is meant the susceptibility of an asphalt cement to variations in viscosity or penetration (ASTM D-5) with temperature, wherein low levels of temperature susceptibility lead to excellent rheological and flow characteristics of the asphalt. The term, "viscosity," is well-known and is the ratio between the applied shear stress and rate of shear and is thus, a measure of the resistance to flow of the liquid. Te term, "penetration," is well-known and is the consistency of a bituminous material expressed as the distance in tenths of a millimeter that a standard needle vertically penetrates a sample of the material under known conditions of loading, time, and temperature. The term, "ductility,"is well-known and is a measure of the elasticity of the asphalt cement, as determined by ASTM D 113-79. High ductility leads to improved resistance to cracking and disbonding. Thus, an acceptable asphalt material should have a low level of temperature susceptibility and high ductility.
A further factor in light of the current energy shortage is that the use of a straight run residuum is being discouraged since the residuum still contains considerable amounts of lubricating oil fractions which are often more valuable in other applications than in asphalt paving formulations. Consequently, liquid oil fractions are additionally solvent-extracted from the residuum, following distillation, to maximize lubricating oil and fuel product yields.
One process in the art that is currently used is the treatment of a residuum with propane to extract heavy gas oil fractions yielding a liquid propane-extracted oil fraction and a solid propane-precipitated asphalt, termed "PPA." The propane-extracted oil fraction can then further be extracted with furfural or phenol, for example, to yield a raffinate suitable for making lube oils. The remaining liquid material, after removal of solvent, is an extract fraction called "extract oil." The extract oil can be blended, i.e., "fluxed" with the solid PPA obtained from the propane treatment, to form an asphalt cement composition with improved temperature susceptibility and ductility characteristics. These characteristics can be further improved by subjecting the PPA to "air-blowing," i.e., treating the mixture with an air stream at about 250.degree. to 300.degree. C. for a period of time to increase the softening point and temperature susceptibility of the material. However, this "air-blown" material, sometimes termed "semi-blown" material, prepared from a PPA derived from a light residuum, and fluxed with straight extract oil, is still marginal with respect to meeting temperature susceptibility and ductility requirements of ASTM D 3381-76 and AASHTO M 226-73. What is desired is a process which will further improve these properties toward meeting specifications.
Currently, there are paving formulations being used whch incorporate elemental sulfur as a dispersion to improve its temperature susceptibility characteristics. However, a contractor who buys the base and mixes it with mineral aggregate, prior to use, must heat the material in large quantities to achieve a workable consistency, thus periodically generating sulfurous fumes. What is desired is an asphalt base or cement formulation having improved temperature susceptibility and ductility characteristics which does not emit noxious or toxic sulfur-containing fumes upon mixing with mineral aggregate or application to road surfaces.
Sulfur-treating of bituminous material is known in the art. For example, sulfur-treating of propane-precipitated asphalt (PPA) to improve the blending properties of the asphalt in bitumens is known and is described in British Patent No. 956,313. The patent describes heating PPA with 2-20 weight percent elemental sulfur at 130.degree. to 300.degree. C. for 2 to 10 hours to increase the asphaltene content and reduce the wax content of the material. However, no specific mention is made of the combined steps of completely reacting the PPA derived from a residuum from a light crude with sulfur, and blending the resulting base material with an extract oil to obtain an asphalt cement composition capable of meeting specifications.
British Patent No. 610,629 describes the production of improved bituminous asphalt material in which a mixture of an asphalt, an unsaturated vegetable oil, e.g., linseed oil, and sulfur is heated at about 175.degree. to 180.degree. C. until a smooth, homogeneous bubble-free molten mass is obtained. The resulting material is described as exhibiting good thermal stability, and good susceptibility to plastic flow. However, the process requires the presence and added expense of an unsaturated vegetable oil for obtaining good rheological properties.
U.S. Pat. No. 3,317,447 (1967) discloses a process for treating asphaltenes (a minor fraction of asphalt) derived from crude oil residua. The asphaltenes are heated with an alpha olefin polymer in the presence of elemental sulfur at about 150.degree. to 540.degree. C. to yield a reaction product having a melting point in the range of about 130.degree. to 315.degree. C. The reference states that asphaltic compositions produced by heating asphaltenes with sulfur alone are inferior compositions for many applications.
What is desired is a convenient and inexpensive process for preparing asphalt cement compositions having excellent temperature susceptibility and ductility characteristics, from a propane-precipitated asphalt, which does not require treating the material with unsaturated vegetable oils or organic polymers. Particularly what is desired is a process for converting propane-precipitated asphalt, derived from a light crude oil, into a high quality asphalt cement composition of ASTM grades AC-21/2 , AC-5, AC-10, AC-20 and AC-40, which meet the specifications of ASTM D 3381-76 (Table 2) or AASHTO M 226-73 (Table II).